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Monday, July 11, 2011

LCR Phonostage With Variable EQ Curves, Part 1: Initial Thoughts

Hi!

I often have been asked if I could develop a phonostage based on my circuit philosophy but with variable EQ curves for mono records. I'm starting such a project now. The first one will be a mono phonostage, transformer coupled using two stages with EC8020. The same circuit as the recently presented EC8020 LCR RIAA phonostage.

EQ will be adjustable by two separate selector switches for the bass EQ (turnover frequency and bass boost) and the treble cut (which selects the -3dB corner frequency of the treble EQ). This first variable EQ phonostage will be based on my best sounding phono circuit, using LCR networks. This means that there need to be separate LCR networks for the bass and treble EQ, since these are indivisually selectable. This is no problem since there are no integrated LCR modules available anyways for other curves than RIAA. So the networks need to be implemented discretely anyways.

It seems that there had been a whole lot of different EQ curves before the RIAA standard was established. Trying to catch all possible curves with selectable LCR networks will be impossible since the LCR filters are quite big and expensive. Therefore I need to find a set of bass and treble EQ networks which provide a useful range which should cover a reasonable range of mono records. 4 to 5 settings maximum seem to be feasable for both bass and treble EQ. Since both can be selected individually this will yield quite a view useful combinations.

After some research I came up with these EQ networks:

Bass EQ, turnover frequency and low freq cut:

1. 350 / -
2. 400 / -
3. 500 / 50
4. 500 / 100

With LCR networks it would be impractical to have the bass boost increasing down to 0 as it would require coils of infinite inductance. So for practical reasons there will be a shelf introduced at around 25Hz for settings 1 and 2.

Treble EQ, high frequency cut, -3dB point:

1. 1590
2. 2120
3. 3180
4. 5000
5. flat

There are other frequencies around. Some of them are quite close to frequencies in the list above, so as an approximation the nearest setting could be used as a compromise.

These are my initial thoughts on the EQ curves. I would appreciate feedback from people with knowledge in this field. Would you consider this selection as being reasonably representative? Keep in mind this is to cover the most mono records of the 40ies and 50ies. Exotic and rarely used curves for really old 78s are not required for this project.

Hello Thomas,For years I have been following your efforts in the web i.e. preamp / anode chokes / inductive RIAA / DHT and an expense that is unique here. First of all you are doing an exceptional job as far as I can tell. I'm always stumbling over your posts in various forums within these years, since I'm building itself in the issues inductive RIAA / variable and also with DHT. Have you tried that with DHT and if anything, the experience here? I even try that with a double Posttriode in the bodywork, but no finished using RIAA module but a similar solution with variable Sakuma UTC coils.

I have built various different LCR RIAAs so far, both with integrated as well as discrete networks. The Sakuma approach with a tunable UTC coils is not very exact, it does not follow the RIAA response, so be careful with that. He intentionally uses it to create a certain sound.I have also built phonostages with DHT. This is doable but requires a lot of effort. You need to control the microphonics. I have used UX201A and 801A/10Y in phonostages.

I know the problems with the tunable coils, but it makes as sense bejond the argument of correctnes. You can use different coils switched to get the one half of the equalisation curve without resitors, which is a big succes in fine dynamics. Apart from that it gives you a lot of freedom for the tube choices in the first stage.I wanted the possibility of different EQ-curves because I listen foremost to first release jazz records of the early fifties to mid sixties. I hoped to compensate the differences between the brands of records. But the everyday practice shows that the differences are more determined through different recording equipment, studio acustics than from record equalisation. So might be doing more sense with the older 78 records, where the differences are huge.

I examined the EQ of Sakuma closely and it is far away from RIAA or any other curve, whichever setting of the coil it is only a rough approximation. The purpose of this mono phonostage is to have exact EQ curves for a set of labels. In my experience LCR EQ networks are the best sounding. That's why they have been chosen for this project.

I just now discovered your blog - I was looking for a photo of a metal 6N7 on Google Images (to illustrate a comment I made on Blackie's Facebook page) and what did I find but the Vinylsavor blog! Hooray!

Now that I am here I promise to make a pest of myself.

I made a phono preamp for disc restoration engineer Doug Pomeroy, who uses it to make disc transfers for subsequent digital processing and click removal. It has selectable turnover frequencies (I think I made it 200/300/500?) and switchable on/off RIAA rolloff. Not so much designed for listening exactly - Doug will match the turnover to the disc by ear and then (almost always) leaves the HF rolloff set to "off." In this way he captures the maximum click & pop signal for his digital software to find and remove. Then he adds the HF rolloff later, usually in the digital domain, usually by ear. He is mostly restoring 78's which have pretty random cutting characteristics anyway.

I know this is not an idea that would appeal to you, but it is an example of one experienced engineer's method anyway. And it is an example of a way your planned preamp might be used that you might not have thought of.

Doug just finished transferring the William Savory collection for the Jazz Museum of Harlem using this preamp - I'm very proud of this of course! If you look here at this link, he even gives me some credit in an interview, which is extremely kind of him.

It's just a crappy low-impedance RC 6C45P phono that you wouldn't give the time of day, of course, but he thinks it's the best he's used and he likes the functionality which I designed and made specially for him.

I am glad to find your home base.

Don't you own some adjustable UTC inductors? Aren't they in my basement?